The following publications are cited as being close prior art:
EP-A-0 142 488=D1, PA0 EP-A-0 143 100=D2, PA0 EP-A-0 330 527=D3 and PA0 WO-A-94/28717 = EP 93 924 374=D4.
A similar type of method for the isolation of MHA is known, for example, from D1. D1 employs two-step hydrolysis using sulphuric acid in order to isolate MHA in liquid form as a highly-concentrated aqueous solution.
According to D1, MHA is obtained after the hydrolysis reaction, which is carried out via the amide step using excess mineral acid under specified conditions of concentration and temperature, by means of a solvent extraction wherein use is made of certain solvents which are partially miscible with water.
According to the information in D1, the characterising feature of the method described in that document is to be seen in the isolation of MHA from the extraction solution, which is carried out in such a way that the isolation includes the removal of the organic solvent in the presence of at least about 5 wt. % of water, referred to the remaining extract (MHA). MHA is isolated from the extraction solution by distillation (see Examples), with steam distillation being preferred. As a result of removing the solvent from the extraction solution during the steam distillation, the discharge obtained is a mixture of MHA and water. The steam distillation is therefore carried out in such a way that the discharge contains at least 5 wt. % of water.
At another place in the text of D1 it is stated that the column conditions during the distillation are controlled in such a way that everywhere in the column, but at least in the bottom fraction, the liquid phase contains 5 wt. % of water.
It follows from this that in the absence of a sufficient quantity of water during the isolation of MHA from the extraction solution, the increasing formation possibly of undesirable by-products (dimers and oligomers) is to be expected.
Furthermore, in the distillation the steam serves as an operative agent for the complete removal of the extracting agent from the MHA solution, for example, through the formation of a low-boiling azeotropic mixture with the respective extracting agent.
Further embodiments of a method essentially similar in type are described in D2. In contrast to the preamble in D1, the hydrolysis of the MMP-CH by a mineral acid is described, with reference being made to the alternative possibility of using HCl instead of H.sub.2 SO.sub.4. Altogether three additional variants are disclosed, which are concerned essentially with variations in the method of isolating MHA from the mineral acid hydrolysate or in the working up of the extract during the liquid/liquid extraction.
In an initial aspect according to D2, the hydrolysate is brought into contact with the organic solvent without previous separation of any essential fractions from solid substances contained therein. Moreover, according to D2 the conditions of the extraction are controlled so that the extract and the aqueous raffinate are the only liquid phases, which are formed during phase separation after the extraction.
A disadvantage of this first aspect is that the extraction is laden with the whole of the salt component formed in the hydrolysate during hydrolysis, which leads to a relatively high mass flow of hydrolysate and accordingly also of solvent. This results in correspondingly high energy costs in the solvent evaporation and condensation and costs of corresponding loss of solvent and of correspondingly large units for extraction and evaporation. A lowering of the operating and investment costs at this point in the process would therefore be desirable (especially in view of the size of such a plant and the potential for economy associated therewith).
The raffinate obtained from the extraction as a homogeneous liquid phase must, according to D1 or D2 or D4, be freed from remains of solvent by stripping or distillation, which is an undesirable additional expense.
In a second aspect, D2 refers to the separation of the organic solvent from the extract. On this point, it is stated that the separation is effected by subjecting the extract to a steam distillation, with the solvent being distilled off and a bottom fraction of aqueous MHA being formed.
A disadvantage of the use of steam is in particular the increased accumulation of aqueous solvent-laden steam condensate, which has subsequently to be freed from solvent at undesirable additional expense, such as that of distillation or stripping, in order then to return it to the process at a suitable point, or otherwise it has to be expensively disposed of, for example, by burning. Avoidance of additional stripping steam would therefore be desirable.
Finally, in a third aspect, D2 lays emphasis on the nature of the solvent to be used for the liquid/liquid extraction. The criteria to be considered in the selection of a suitable solvent include in particular the following points:
the boiling point of the solvent is to be between 60.degree. C. and 200.degree. C.; PA1 the distribution coefficient for MHA in equilibrium between hydrolysate and solvent is to be at least approximately two; PA1 the distribution coefficient of the solvent in equilibrium between extract and aqueous phase is to be at least approximately one; PA1 the solubility of water in the solvent at room temperature is to be not more than about 12 wt. %. PA1 The evaporation otherwise appropriate prior to the introduction into a split-contact plant can be completely omitted. PA1 More simply constructed and hence cheaper equipment is adequate for a concentration prior to the extraction because, owing to its particular composition, the concentrated solution is very much less aggressive at this particular point in the procedure and is therefore, in particular, also less corrosive. PA1 In the case of at least partial shifting of the evaporation to a particular earlier point in the procedure there is an overall improved coordinated use of energy. Since already tempered (hot) hydrolysate is evaporated, and not raffinate which has already been cooled by extraction, the energy consumption is less. PA1 In the course of a concentration by evaporation, the separation of unwanted low-boiling components of the hydrolysate is improved. PA1 The hydrolysis of the MHA amide can be carried out in dilute solution, which results in a more complete chemical conversion. As it is better, in the hydrolysis of MHA amide using sulphuric acid, to employ a more dilute sulphuric acid (&lt;40 wt. %) in order to allow the hydrolysis to take place as completely as possible, a less highly concentrated sulphuric acid can be used in the hydrolysis of the MHA amide, without its being necessary to anticipate that the subsequent extraction with an organic solvent will proceed less favourably. Owing to the concentration, a deterioration of the distribution coefficient is avoided and, in particular, less MHA remains in the raffinate during the extraction.
The relatively high boiling point range, of from 60 to 200.degree. C., of the solvents to be used here necessitates for the evaporation of the extract relatively elevated temperatures, which may impair the product, as well as additional auxiliaries, such as stripping steam, which is undesirable.
One of the considerable disadvantages of the methods in D1 and D2 consists however in the high salt content, which forms during the saponification and which nevertheless unfavourably contaminates the method for the isolation of MHA in an otherwise relatively elegant liquid/liquid extraction process. A working up of the inevitably resulting mixture of ammonium salts is in most cases not economic, the disposal is very hazardous from the environmental aspect and in the foreseeable future is likely to be prohibited by law even at sites having few rigid conditions.
There has been no lack of attempts to lessen or even to avoid the accumulation of salt from the saponification, but the advantages gained thereby were in every case achieved by accepting a number of other disadvantages or by dispensing with the elegant handling according to D1 and D2.
Thus in D3 there is disclosed a single-step method of hydrolysis using sulphuric acid as saponifying agent, which is carried out without solvent and leads directly to concentrated aqueous MHA solutions, with crystalline ammonium sulphate in marketable form being obtained as the coproduct. This object is achieved by neutralising the saponification mixture with ammonium hydroxide solution to the extent that the excess mineral acid and the ammonium bisulphate formed are converted into the neutral sulphate with the formation of two liquid phases, which for their part are separated and evaporated in order to isolate firstly liquid MHA and secondly crystalline ammonium sulphate. During this the different filtration and recirculation steps are combined in such a way that virtually no product is lost and no waste water contaminated with salt is formed. The resulting MHA is of a quality similar to that of the product obtained in D1.
However, even this relatively environmentally harmless method has various disadvantages. When reproducing this method, the Applicant of the present invention found that firstly, because of the comparatively high dilution of the sulphuric acid (20-50%), excesses of acid definitely higher than those given have to be used in order to achieve a complete cyanohydrin conversion. Moreover, to avoid precipitations of salt during neutralisation the method has to be carried out at higher dilution, to render possible a clean separation of the two liquid phases. Secondly, the ammonium sulphate isolated has a sticky consistency and is tainted with a strong smell, so that an aftertreatment such as, for example, a washing filtration or recrystallisation appears unavoidable and the method is thereby additionally made more expensive. Moreover the method--unlike what is postulated--consumes more energy in the evaporation steps than does the method cited by way of comparison in D1. Further, it is cost-intensive and very expensive as regards apparatus for the treatment of solids using filtration/centrifugation, which involves two separate paths, as well as the drying of the ammonium sulphate, which is not shown in the flow diagram.
A partial solution to the dilemma is promised by D4. D4 discloses the recovery of sulphuric acid from a sulphate-containing flow of waste material which arises during the preparation of 2-hydroxy-4-(methylthio)butyric acid by hydrolysis of 2-hydroxy-4-(methylthio)butyronitrile using sulphuric acid.
The recovery of sulphuric acid from ammonium sulphate, ammonium bisulphate and/or residues containing sulphuric acid has for a long time been prior art in the preparation of MMA and, just as is known for the residues from the saponification of acetone cyanohydrin, is achieved by combustion in a so-called split-contact plant of the flows of waste matter arising during saponification and extraction.
Here, in a manner familiar to the person skilled in the art, SO.sub.2 is first of all produced as a decomposition product, which is oxidised on the contact catalyst to form SO.sub.3, which is finally converted into sulphuric acid. The resulting sulphuric acid can then be returned again to the saponification process, while the other former constituents of the "load of salt" may be found substantially in the form of combustion gases.
Elegant as this method may be, it is also not free from disadvantages. Thus in the methods according to D1 and D2 flows of waste material arise, whereof the sulphate concentration is relatively low, but invariably too low to permit direct introduction into a split-contact plant. Hence a concentration or increase by mixing with concentrated flows of waste material from other processes is generally essential. Conventional solutions employed for the operation of split-contact plants have a sulphate salt content of &gt;50 wt. %. Higher concentrations are even more preferred. Concentration by evaporation of the waste water arising from the isolation of MHA is however, owing to the high corrosiveness of the waste water, a relatively costly undertaking which, from the selection of special materials for the evaporation equipment to the special safety precautions required, is excessive and expensive.